Severe turbulence such as the kind that injured 21 people Tuesday aboard a United Airlines flight is the leading cause of serious midair injuries outside of fatal crashes, federal accident statistics show.

Nearly all of the serious injuries — including broken bones and spinal damage — happened when people were not wearing their seat belts, according to the federal statistics.

"This just reinforces how critically important it is to stay seated during the flight and belted in," said David Castelveter, spokesman for the Air Transport Association, which represents large air carriers.

United Flight 967, a Boeing 777 flying from Washington, D.C.'s, Dulles International to Los Angeles, encountered a severe jolt of unsteady air over western Missouri at 7:14 p.m., said Federal Aviation Administration (FAA) spokesman Ian Gregor.

Some passengers were tossed around the plane like dolls, passenger Kaoma Bechaz, 19, told The Denver Post. One woman's head struck the side of the cabin, leaving a crack above the window, and a girl was flung against the ceiling, Bechaz said.

Seventeen passengers and four flight attendants were taken to Denver-area hospitals after the pilot diverted there, the National Transportation Safety Board (NTSB) said. None were seriously injured.

The flight was flying near thunderstorms, and pilots had been alerted by air-traffic controllers about the weather, Gregor said. The pilots had warned passengers about the possibility of turbulence and switched on the seat belt sign, said United spokeswoman Megan McCarthy.

Turbulence can create violent bucking, making passengers and heavy beverage carts weightless, then slamming them onto walls, ceilings or the floor.

A woman was paralyzed on April 18, 2009, in turbulence on a Continental Airlines flight over Texas. She had gone to the lavatory despite the seat belt sign and public address announcements warning passengers to stay seated, the NTSB concluded.

Out of 22 people who suffered serious injuries such as broken bones on flights last year, 15 of them, or 68%, were caused by turbulence, according to NTSB data.

From 1980 through 2008, three people died and 298 were seriously hurt in turbulence, the FAA reported. All but four of them had not been belted in, the agency said.

Flight attendants, who must move around the cabin more than passengers, are at far higher risk of being injured. Even though there are only a handful of flight attendants on each flight, they suffered 62% of serious injuries, according to the FAA study.

The Association of Flight Attendants-Communications Workers of America, which represents flight attendants at 22 airlines, negotiates provisions to guarantee that flight attendants receive additional benefits if they are injured in turbulence, spokeswoman Corey Caldwell said.

The FAA and airlines have tried for years to encourage passengers wear seat belts even when the seat belt sign is not lit. Most airlines tell their pilots to urge passengers to remain belted in.

Still, many passengers ignore the seat belt sign. "You can see the frustration of cabin crews trying to enforce the seat belt sign," said Bill Voss, president of the Flight Safety Foundation. "You have to take the seat belt sign seriously."

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